The prior art most closely related to that of the present invention is that covered by U.S. Pat. No. 5,328,309 (Bishop et al), U.S. Pat. No. 5,292,214 (Bishop et al). These prior art references relate to "slotting machines" which machine the slots in the bore of power steering valve sleeves. The design of these machines calls for each of the slots to be scooped out of the bore of a turned blank by a finger-like cutting tool mounted on a cutting spindle which angularly reciprocates about an axis in a series of progressively deeper cutting and return strokes so forming slots in the form of (usually) closed chambers, which are arcuate or partially arcuate in longitudinal section. The sleeve is held in a work holding collet, in turn mounted in a work holding spindle, having a rotational axis perpendicular to and offset with respect to the cutting spindle axis. By accurately indexing the work holding spindle upon completion of each slot, the required number of slots are precisely machined in the sleeve, usually 4, 6, 8, 10 or 12 slots for most automotive applications.
A problem associated with slotting is a tendency for bore material to burr at the edges of each slot being machined. Once an array of slots have been machined in the bore of a sleeve, a bore finishing operation can be performed by use of a boring tool. Such a method is disclosed in U.S. Pat. No. 5,390,408 (Bishop et al). However in this operation, the boring tool has been found to push the burrs on the slot edges in the direction of rotation, resulting in burr material overhanging each slot from one of its edges. On a typical slot width of say 4.5 mm, the burr overhang size may for instance be in the order of 30-70 .mu.m.
A problem with slot edge burrs is that they affect the "boost control" and increase the noise generated during operation of the power steering valve. It has been found that the aforesaid limitations on slot machining of sleeves have imposed limitations on design of steering gear valves, and an object of the invention is to alleviate the situation by minimising the resultant burr on the sleeve edges.